Asymmetric tibial components for a knee prosthesis

ABSTRACT

An orthopaedic tibial prosthesis includes a tibial baseplate with an asymmetric periphery which promotes proper positioning and orientation on a resected tibia, while also facilitating enhanced kinematics, soft-tissue interaction, and long-term fixation of the complete knee prosthesis. The asymmetric baseplate periphery is sized and shaped to substantially match portions of the periphery of a typical resected proximal tibial surface, such that proper location and orientation is evident by resting the baseplate on the tibia. The baseplate periphery provides strategically positioned relief and/or clearance between the baseplate periphery and bone periphery, such as in the posterior-medial portion to prevent deep-flexion component impingement, and in the anterior-lateral portion to avoid undue interaction between the anatomic iliotibial band and prosthesis components.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under Title 35, U.S.C. §119(e) ofU.S. Provisional Patent Application Ser. No. 61/381,800, filed on Sep.10, 2010 and entitled TIBIAL PROSTHESIS FACILITATING ROTATIONALALIGNMENT, and U.S. Provisional Patent Application Ser. No. 61/367,375,filed on Jul. 24, 2010 and entitled TIBIAL PROSTHESIS, the entiredisclosures of which are hereby expressly incorporated by referenceherein.

BACKGROUND

1. Technical Field

The present disclosure relates to orthopaedic prostheses and,specifically, to tibial components in a knee prosthesis.

2. Description of the Related Art

Orthopaedic prostheses are commonly utilized to repair and/or replacedamaged bone and tissue in the human body. For example, a kneeprosthesis may include a tibial baseplate that is affixed to a resectedor natural proximal tibia, a femoral component attached to a resected ornatural distal femur, and a tibial bearing component coupled with thetibial baseplate and disposed between the tibial baseplate and femoralcomponent. Knee prostheses frequently seek to provide articulationsimilar to a natural, anatomical articulation of a knee joint, includingproviding a wide range of flexion.

The tibial insert component, sometimes also referred to as a tibialbearing or meniscal component, is used to provide an appropriate levelof friction and contact area at the interface between the femoralcomponent and the tibial bearing component. For a knee prosthesis toprovide a sufficient range of flexion with a desirable kinematic motionprofile, the tibial bearing component and tibial baseplate must be sizedand oriented to interact appropriately with the femoral component of theknee prosthesis throughout the flexion range. Substantial design effortshave been focused on providing a range of prosthesis component sizes andshapes to accommodate the natural variability in bone sizes and shapesin patients with orthopaedic prostheses, while preserving flexion rangeand desired kinematic motion profile.

In addition to facilitating implantation and providing enhancedkinematics through manipulation of the size and/or geometry ofprosthesis components, protection and/or preservation of soft tissues inthe natural knee joint is also desirable.

A given prosthetic component design (i.e., a tibial baseplate, tibialbearing component, or femoral component) may be provided to a surgeon asa kit including a variety of different sizes, so that the surgeon maychoose an appropriate size intraoperatively and/or on the basis ofpre-surgery planning. An individual component may be selected from thekit based upon the surgeon's assessment of fit and kinematics, i.e., howclosely the component matches the natural contours of a patient's boneand how smoothly the assembled knee joint prosthesis functions inconjunction with adjacent soft tissues and other anatomical structures.Soft tissue considerations include proper ligament tension andminimization of soft tissue impingement upon prosthetic surfaces, forexample.

In addition to prosthetic sizing, the orientation of a prostheticcomponent on a resected or natural surface of a bone also impactssurgical outcomes. For example, the rotational orientation of a tibialbaseplate and tibial bearing component with respect to a resectedproximal tibia will affect the interaction between the correspondingfemoral prosthesis and the tibial bearing component. The nature andamount of the coverage of a tibial baseplate over specific areas of theresected proximal tibia will also affect the fixation of the implant tothe bone. Thus, substantial design efforts have been focused onproviding prosthetic components which are appropriately sized for avariety of patient bone sizes and are adapted to be implanted in aparticular, proper orientation to achieve desired prosthesis performancecharacteristics.

SUMMARY

The present disclosure provides an orthopaedic tibial prosthesisincluding a tibial baseplate with an asymmetric periphery which promotesproper positioning and orientation on a resected tibia, while alsofacilitating enhanced kinematics, soft-tissue interaction, and long-termfixation of the complete knee prosthesis. The asymmetric baseplateperiphery is sized and shaped to substantially match portions of theperiphery of a typical resected proximal tibial surface, such thatproper location and orientation is evident by resting the baseplate onthe tibia. The baseplate periphery provides strategically positionedrelief and/or clearance between the baseplate periphery and boneperiphery, such as in the posterior-medial portion to preventdeep-flexion component impingement, and in the anterior-lateral portionto avoid undue interaction between the anatomic iliotibial band andprosthesis components.

In one form thereof, the present invention provides a method ofperforming a tibial arthroplasty, the method comprising: resecting aproximal end of a tibia to create a resected tibial surface, the tibiahaving an anteriorly disposed tibial tubercle and a connection point toa posterior cruciate ligament; identifying an anatomic anteroposterioraxis which intersects: an anterior point disposed on a tubercle of thetibia, the anterior point medially spaced from a tubercle peak by anamount equal to ⅙^(th) of an overall mediolateral width of the tuberclesuch that the anterior point lies on the medial third of the tubercle;and a posterior point disposed at a geometric center of an area definedby the connection point of the tibia to the posterior cruciate ligament;providing a tibial prosthesis having alignment indicia indicating acomponent anteroposterior axis designed to be aligned with the anatomicanteroposterior axis; and positioning and orienting the tibialprosthesis with respect to the tibia by aligning the alignment indiciawith the anatomic anteroposterior axis.

In another form thereof, the present invention provides a method ofimplanting a tibial prosthesis on a tibia, the method comprising:providing a tibial prosthesis comprising: a distal surface; a proximalsurface generally opposite the distal surface; and a peripheral wallextending between the distal surface and the proximal surface, theperipheral wall defining an anterior edge; a PCL cutout area generallyopposite the anterior edge; a medial compartment, and a lateralcompartment generally opposite the medial compartment; and ananteroposterior axis bisecting the anterior edge and bisecting the PCLcutout area, the medial compartment cooperating with the anteroposterioraxis to bound a medial surface area, the lateral compartment cooperatingwith the anteroposterior axis to bound a lateral surface area, themedial surface area larger than the lateral surface area; and aligningthe anteroposterior axis to an anatomic home axis, the anatomic homeaxis defined as a line extending from a posterior point at a geometriccenter of an attachment area between a posterior cruciate ligament andthe tibia to an anterior point disposed on an anterior tubercle of thetibia, the tubercle having a tubercle width W, the anterior pointdisposed on the tubercle at a location medially spaced from a peak ofthe tubercle by an amount equal to W/6.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this invention,and the manner of attaining them, will become more apparent and theinvention itself will be better understood by reference to the followingdescription of embodiments of the invention taken in conjunction withthe accompanying drawings, wherein:

FIG. 1A is an exploded, perspective view of a tibial baseplate andtibial bearing component in accordance with the present disclosure;

FIG. 1B is an assembled, perspective view of the tibial baseplate andtibial bearing component shown in FIG. 1A;

FIG. 2A is a top plan view of the peripheries of a set of nine tibialbaseplates made in accordance with the present disclosure, in which theperipheries are shown to scale according to the illustrated scales inmillimeters in the bottom and right-hand margins of the page;

FIG. 2B is a top plan view of the periphery of a tibial baseplate madein accordance with the present disclosure;

FIG. 2C is a graph illustrating the asymmetric growth of theposterior-medial compartment for the tibial baseplates shown in FIG. 2A;

FIG. 2D is a graph illustrating the asymmetric growth of theposterior-lateral compartment for the tibial baseplates shown in FIG.2A;

FIG. 3A is top plan view of a periphery of a tibial baseplate made inaccordance with the present disclosure, illustrating various arcsdefined by the periphery;

FIG. 3B is a partial, top plan view of the periphery shown in FIG. 3A,illustrating an alternative lateral corner periphery;

FIG. 3C is a partial, top plan view of the periphery shown in FIG. 3A,illustrating an alternative medial corner periphery;

FIG. 3D is a top plan view of the periphery of a tibial baseplate madein accordance with the present disclosure, illustrating medial andlateral surface area calculations without a PCL cutout;

FIG. 4A is a top plan view of a tibial baseplate made in accordance withthe present disclosure;

FIG. 4B is a side elevation view of the tibial baseplate shown in FIGS.4A;

FIG. 5 is a top plan view of a resected proximal tibial surface with aprosthetic tibial baseplate component and tibial bearing component madein accordance with the present disclosure mounted thereon;

FIG. 6 is a top plan view of a resected proximal tibial surface with aproperly sized tibial trial component thereon;

FIG. 7 is a side, elevation view of the tibia and trial component shownin FIG. 6; and

FIG. 8 is a side, elevation view of the tibial components shown in FIG.1A, in conjunction with a femoral component.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate exemplary embodiments of the invention, and suchexemplifications are not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION

The present disclosure provides an asymmetric knee joint prosthesiswhich facilitates proper rotational and spatial orientation of a tibialbaseplate and tibial bearing component upon a resected proximal tibia,while also offering large-area contact with the resected proximal tibia.The prosthesis permits a wide range of flexion motion, protects naturalsoft tissue proximate the knee joint prosthesis, and optimizes long termfixation characteristics of the prosthesis.

In order to prepare the tibia and femur for receipt of a knee jointprosthesis of the present disclosure, any suitable methods orapparatuses may be used. As used herein, “proximal” refers to adirection generally toward the torso of a patient, and “distal” refersto the opposite direction of proximal, i.e., away from the torso of thepatient.

As used herein, the “periphery” of a tibial prosthesis refers to anyperiphery as viewed in a top plan view, e.g., in a generally transverseanatomical plane. Alternatively, the periphery of a tibial prosthesismay be any periphery as viewed in bottom plan view, e.g., in a generallytransverse plane and looking at the distal surface adapted to contact aresected proximal surface of a tibial bone.

As used herein, the term “centroid” or “geometric center” refers to theintersection of all straight lines that divide a given area into twoparts of equal moment about each respective line. Stated another way, ageometric center may be said to be the “average” (i.e., arithmetic mean)of all points of the given area. Stated yet another way, the geometriccenter is a point in a two dimensional figure from which the sum of thedisplacement vectors of all points on the figure equals zero.

As used herein, a “disparity” or “difference” between two numericalvalues (e.g., one value “larger” or “smaller” than another), typicallyexpressed as a percentage, is the difference between the two valuesdivided by the smaller of the two values. For example, a smallerquantity having value 75 and a larger quantity having value 150 wouldhave a percentage disparity of (150−75)/75, or 100%.

Referring to FIG. 5, tibia T includes tibial tubercle B havingmediolateral width W, with tubercle midpoint P_(T) located on tubercle Bapproximately halfway across width W. While tubercle B is shown ashaving midpoint P_(T) at the “peak” or point of maximum anterioreminence, it is recognized that midpoint P_(T) of tibia T may be spacedfrom such a peak. Tibia T also includes attachment point C_(P)representing the geometric center of the attachment area between theanatomic posterior cruciate ligament (PCL) and tibia T. Recognizing thatthe PCL typically attaches to a tibia in two ligament “bundles,” one ofwhich is relatively anterior, lateral and proximal and the other ofwhich relatively posterior, medial and distal, attachment point C_(P) iscontemplated as representing the anterior/lateral attachment area in anexemplary embodiment. However, it is contemplated that theposterior/medial attachment area, or the entire attachment area, couldbe used.

As used herein, “anterior” refers to a direction generally toward thefront of a patient. “Posterior” refers to the opposite direction ofanterior, i.e., toward the back of the patient.

In the context of patient anatomy, “home axis” A_(H) refers to agenerally anteroposterior axis extending from posterior point C_(P) toan anterior point C_(A), in which anterior point C_(A) is disposed ontubercle B and medially spaced from tubercle midpoint P_(T) by an amountequal to W/6. Stated another way, anterior point C_(A) is laterallyspaced by an amount equal to W/3 from the medial end of mediolateralwidth W, such that point C_(A) lies on the “medial third” of theanterior tibial tubercle.

In the context of a prosthesis, such as tibial baseplate 12 describedbelow, “home axis” A_(H) refers to an axis oriented with respect tobaseplate 12 such that the baseplate home axis A_(H) of baseplate 12 isaligned with home axis A_(H) of tibia T after implantation of baseplate12 in a proper rotational and spatial orientation (as shown in FIG. 5).In the illustrative embodiments shown in FIG. 3 and described in detailbelow, home axis A_(H) bisects PCL cutout 28 at the posterior edge ofperiphery 200 of tibial plateau 18 (FIG. 5), and bisects anterior edge202 at the anterior edge of periphery 200 of tibial plateau 18. It iscontemplated that home axis A_(H) may be oriented to other baseplatefeatures, it being understood home axis A_(H) of baseplate 12 ispositioned such that that proper alignment and orientation of baseplate12 upon tibia T positions the home axis A_(H) of baseplate 12 coincidentwith home axis A_(H) of tibia T.

Home axis A_(H) of tibial baseplate 12 may be said to be ananteroposterior axis, as home axis A_(H) extends generally anteriorlyand posteriorly when baseplate 12 is implanted upon tibia T. Tibialbaseplate also defines mediolateral axis A_(ML), which lies along thelongest line segment contained within periphery 200 that is alsoperpendicular to home axis A_(H) of baseplate 12. As described below,home axis A_(H) and mediolateral axis A_(ML) cooperate to define acoordinate system useful for quantifying certain baseplate features inaccordance with the present disclosure.

The embodiments shown and described with regard to FIGS. 1A, 1B, 3A, 4A,4B, 5 and 6 illustrate a left knee and associated features of aright-knee prosthesis, while the embodiments shown and described inFIGS. 2A, 2B and 3D illustrate the periphery of a right knee prosthesis.Right and left knee configurations are mirror images of one anotherabout a sagittal plane. Thus, it will be appreciated that all aspects ofthe prosthesis described herein are equally applicable to a left- orright-knee configuration.

1. Asymmetry of the Tibial Prosthesis.

Referring now to FIGS. 1A and 1B, tibial prosthesis 10 includes tibialbaseplate 12 and tibial bearing component 14. Tibial baseplate 12 mayinclude a stem or keel 16 (FIG. 4B) extending distally from proximaltibial plateau 18, or may utilize other fixation structures for securingbaseplate 12 to tibia T, such as distally extending pegs. Portions ofthe outer periphery defined by tibial plateau 18 closely correspond insize and shape with a resected proximal surface of tibia T, as describedin detail below.

Tibial bearing component 14 and tibial baseplate 12 have a particularasymmetry, with respect to home axis A_(H) (shown in FIG. 2A anddescribed above), that is designed to maximize tibial coverage for alarge proportion of knee-replacement candidates. This high level ofcoverage allows a surgeon to cover the largest possible area on theproximal resected surface of the tibia, which in turn offers maximumcoverage of cortical bone. Advantageously, the maximized coverage ofcortical bone facilitates superior support of tibial baseplate 12. Afirm, enduring fixation of tibial baseplate 12 to tibia T is facilitatedby large-area contact between the cortical and cancellous bone of tibiaT and distal surface 35 of tibial plateau 18 (FIG. 4B), which may becoated with porous ingrowth material and/or bone cement.

In an analysis of a several human specimens, variations in size andgeometry for a variety of anatomic tibial features were observed andcharacterized. Geometrical commonalities between anatomic features, orlack thereof, were noted. Mean tibial peripheral geometries werecalculated based on statistical analysis and extrapolation of thecollected anatomical data, in view of the observed geometricalcommonalities organized around anatomic home axis A_(H). Thesecalculated mean geometries were categorized by tibial size.

A comparison between the asymmetric peripheries for the present familyof prostheses and the calculated mean tibial geometries was conducted.Based on the results of this comparison, it has been found thatsubstantial tibial coverage can be achieved for a large proportion ofpatients using tibial components having asymmetric peripheries inaccordance with the present disclosure. Moreover, this coverage can beachieved with a relatively small number of sizes, even where particularportions of the prosthesis periphery is intentionally “pulled back” fromthe tibial periphery in order to confer other orthopaedic benefits.Further, the particular asymmetry of tibial baseplate 12 can be expectedto offer such coverage without overhanging any portion of the resectedsurface.

Thus, periphery 200 including the particular asymmetric profile asdescribed below confers the benefits of maximum coverage, facilitationof proper rotation (discussed below), and long-term fixation asdescribed herein. Such asymmetry may be demonstrated in various ways,including: by a comparison of adjacent radii in the medial and lateralcompartments of the asymmetric periphery; by a comparison of the edgelength in anterior-medial and anterior lateral corners of the periphery,for a comparable lateral and medial angular sweep; and by a comparisonof the location of radius centers for the anterior-medial andanterior-lateral corners with respect to a mediolateral axis. Variouscomparisons and quantifications are presented in detail below. Specificdata and other geometric details of the peripheries for the variousprosthesis sizes, from which the below-identified comparisons andquantifications are derived, may be obtained from the draw-to-scaleperipheries shown in FIG. 2A.

Advantageously, the asymmetry of tibial component 12 encourages properrotational orientation of baseplate 12 upon implantation thereof ontotibia T. As described in detail below, the asymmetry of periphery 200(FIG. 2A) of tibial plateau 18 is designed to provide a close match inselected areas of the lateral and medial compartments as compared to theanatomic bone. As such, a surgeon can select the largest possiblecomponent from among a family of different component sizes, such thatthe component substantially covers the resected tibia T with minimalgaps between the tibial periphery and component periphery 200, as wellas little or no overhang over any portions of the tibial periphery.Because the high congruence between prosthesis periphery 200 and thetibial periphery produces only a minimal gap between the peripheries (asshown in FIG. 5), tibial baseplate 12 cannot be rotated significantlywithout causing tibial plateau 18 to overhang beyond the periphery ofthe resected tibial surface. Thus, proper rotation of baseplate 12 canbe ascertained by the visual acuity between prosthesis periphery 200 andthe resected tibial surface.

The following examples and data are presented with respect to tibialbaseplate 12. However, as described in more detail below, tibial bearingcomponent 14 defines perimeter wall 54 which follows peripheral wall 25of baseplate 12 except where noted. Thus, it is appreciated that theconclusions, trends and design features gleaned from data relating tothe asymmetric periphery of tibial baseplate 12 also applies to theasymmetric periphery of tibial bearing component 14, except where statedotherwise.

Lateral compartment 20 and medial compartment 22 of tibial plateau 18are dissimilar in size and shape, giving rise to the asymmetry thereofThis asymmetry is designed so that peripheral wall 25 traces theperimeter of the resected proximal surface of tibia T, such that tibialplateau 18 covers a large proportion of the resected proximal tibialsurface as shown in FIG. 5. To achieve this large tibial coverage,tibial plateau 18 closely matches the periphery of tibia T in most areasas noted above. Nevertheless, as shown in FIG. 5, for example, a smallgap between periphery 200 of tibial plateau 18 and tibia T is formed toallow some freedom of positioning and rotational orientation. The gap isdesigned to have a substantially continuous width in most areas,including the anterior edge, anterior-medial corner, medial edge,lateral edge and lateral-posterior corner (all described in detailbelow).

However, certain aspects of the asymmetric shape are designed tointentionally deviate from the calculated anatomical shape to conferparticular features and advantages in the context of a complete,implanted knee prosthesis. Referring to FIG. 5, for example, tibialbaseplate 12 and tibial bearing component 14 have anterior-lateral“corners” (described in detail below) which are “pulled back” to creategap 56 between tibia T and prosthesis 10 in the anterior-lateral area ofthe resected surface of tibia T. Advantageously, gap 56 creates extraspace for “soft-tissue friendly” edges of prosthesis 10, therebyminimizing impingement of the iliotibial band. In an exemplaryembodiment, gap 56 may range from 0.5 mm for a small-size prosthesis(such as size 1/A described below), to 1 mm for a medium-sizedprosthesis (such as size 5/E described below), to 2 mm for a large-sizedprosthesis (such as size 9/J described below).

Similarly, the posterior edge of the medial compartment may be “pulledback” from the adjacent edge of tibia T to define gap 58. Gap 58 allowsextra space for adjacent soft tissues, particularly in deep flexion asdescribed below. Gap 58 also allows prosthesis 10 to be rotated about alateral pivot by a small amount, thereby offering a surgeon the freedomto displace medial compartment 22 posteriorly as required or desired fora particular patient. In an exemplary embodiment, gap 58 is about 4 mm.

As described in detail below, the asymmetrical periphery also provides alarge overall area for proximal surface 34 of baseplate 12, whichcreates sufficient space for large contact areas between tibial bearingcomponent 14 and femoral component 60 (FIG. 8).

a. Medial/Lateral Peripheral Curvatures

The particular asymmetric shape of tibial plateau 18 (and of tibialbearing component 14, which defines a similar periphery as describedbelow) gives rise to a generally “boxy” or angular periphery in lateralcompartment 20, and a “rounded” or soft periphery in medial compartment22.

Turning to FIG. 3A, the periphery 200 of tibial plateau 18 surroundslateral compartment 20 and medial compartment 22, each of which define aplurality of lateral and medial arcs extending between anterior edge 202and lateral and medial posterior edges 204, 206 respectively. In theillustrative embodiment of FIG. 3A, anterior edge 202, lateral posterioredge 204 and medial posterior edge 206 are substantially planar andparallel for ease of reference. However, it is contemplated that edges202, 204, 206 may take on other shapes and configurations within thescope of the present disclosure, such as angled or arcuate.

In the exemplary embodiment of FIG. 3A, lateral compartment 20 includesfive separate arcs including lateral anterior edge arc 208,anterior-lateral corner arc 210, lateral edge arc 212, posterior-lateralcorner arc 214, and lateral posterior edge arc 216. Each of lateral arcs208, 210, 212, 214 and 216 defines angular sweep 1L, 2L, 3L, 4L and 5L,respectively, having radii R1L, R2L, R3L, R4L and R5L respectively. Aradius of a particular angular sweep extends from the respective radiuscenter (i.e., one of centers C1L, C2L, C3L, C4L and C5L) to periphery200. Radii R1L, R2L, R3L, R4L and R5L each remain unchanged throughoutthe extent of angular sweeps 1L, 2L, 3L, 4L and 5L, respectively.

Similarly, medial compartment 22 includes three separate arcs includinganterior-medial corner arc 220, medial edge arc 222 andposterior-lateral corner arc 224, defining angular sweeps 1R, 2R and 3R,respectively having radii R1R, R2R and R3R respectively.

In FIG. 2A, peripheries 200 _(X) are shown for each of nineprogressively larger component sizes, with 200 ₁ being the periphery ofthe smallest size (size “1” or “A”) and 200 ₉ being the periphery of thelargest size (size “9” or “J”). For purposes of the present disclosure,several quantities and features of tibial baseplate 12 may be describedwith the subscript “X” appearing after the reference numeralcorresponding to a component size as set for in the Tables, Figures anddescription below. The subscript “X” indicates that the referencenumeral applies to all nine differently-sized embodiments described andshown herein.

In exemplary embodiments, medial and lateral radii may be any valuewithin the following ranges: for medial radius R1R_(X), between about 27mm and about 47 mm; for medial radius R2R_(X), between about 21 mm andabout 49 mm; for medial radius R3R_(X), between about 14 mm and about 31mm; for lateral radius R1L_(X), between about 46 mm and about 59 mm; forlateral radius R2L_(X), between about 13 mm and about 27 mm; for lateralradius R3L_(X) between about 27 mm and about 46 mm; for lateral radiusR4L_(X) between about 6 mm and about 14 mm; and for lateral radiusR5L_(X) between about 22 mm and about 35 mm.

In exemplary embodiments, medial and lateral angular extents or sweepsmay be any value within the following ranges: for medial angle 1R_(X),between about 13 degrees and about 71 degrees; for medial angle 2R_(X),between about 23 degrees and about 67 degrees; for medial angle 3R_(X),between about 23 degrees and about 90 degrees; for lateral angle 1L_(X),between about 11 degrees and about 32 degrees; for lateral angle 2L_(X),between about 42 degrees and about 63 degrees; for lateral angle 3L_(X),between about 23 degrees and about 47 degrees; for lateral angle 4L_(X),between about 36 degrees and about 46 degrees; and for lateral angle5L_(X), between about 28 degrees and about 67 degrees;

The unique asymmetry of periphery 200 defined by tibial plateau 18 canbe quantified in multiple ways with respect to the curvatures of lateraland medial compartments 20 and 22 as defined by the arrangement andgeometry of lateral arcs 208, 210, 212, 214, 216 and medial arcs 220,222, 224.

One measure of the asymmetry of periphery 200 is found in a simplecomparison of radii R2L and R1R, which are the anterior “corner” radiiof lateral and medial compartments 20 and 22 respectively. Generallyspeaking, a corner of a baseplate periphery may be said to be thatportion of the periphery where a transition from an anterior orposterior edge to a lateral or medial edge occurs. For example, in theillustrative embodiment of FIG. 3A, the anterior-lateral corner isprincipally occupied by anterior-lateral corner arc 210, which defines asubstantially medial-lateral tangent at the anterior end of arc 210 anda substantially anteroposterior tangent at the lateral end of arc 210.Similarly, the medial corner of periphery 200 is principally occupied byanterior-medial corner arc 220, which defines a substantiallymedial-lateral tangent at the anterior end of arc 220 and a moreanteroposterior tangent at the lateral end of arc 220. For somepurposes, the anterior-medial corner of periphery 200 may be said toinclude a portion of medial edge arc 222, as described below.

A periphery corner may also be defined by a particular angular sweepwith respect to an anteroposterior reference axis. Such reference axismay extend posteriorly from an anterior-most point of a tibialprosthesis (e.g., from the center of anterior edge 202 of periphery 200)to divide the prosthesis into medial and lateral halves. In asymmetrical prosthesis, the anteroposterior reference axis is the axisof symmetry.

In the illustrative embodiment of FIG. 3A, the anteroposterior referenceaxis may be home axis A_(H), such that the anterior-medial corner ofperiphery 200 occupies some or all of the 90-degree clockwise angularsweep between home axis A_(H) (at zero degrees, i.e., the beginning ofthe clockwise sweep) and mediolateral axis A_(ML) (at 90 degrees, i.e.,the end of the sweep). Similarly, the anterior-lateral corner ofperiphery 200 occupies some or all of the 90-degree counter-clockwiseangular sweep between home axis A_(H) and mediolateral axis A_(ML).

For example, the anterior-medial and anterior-lateral corners may eachoccupy the central 45 degree angular sweep of their respective 90-degreeangular sweeps as described above. Thus, the anterior-lateral corner ofperiphery 200 would begin at a position rotated 22.5 degreescounter-clockwise from home axis A_(H) as described above, and would endat 67.5 degrees counter-clockwise from home axis A_(H). Similarly, theanterior-medial corner would begin at a 22.5-degree clockwise rotationand end at a 67.5 degree clockwise rotation.

It is contemplated that the anterior-lateral and anterior-medial cornersmay occupy any angular sweep as required or desired for a particulardesign. For purposes of comparison between two corners in a givenprosthesis periphery, however, a comparable angular sweep for thelateral and medial sides is envisioned, i.e., the extent and location ofthe compared angles may be “mirror images” of one another about ananteroposterior axis. For example, in a comparison of anterior-lateraland anterior-medial radii R2L, R1R, it is contemplated that suchcomparison is calculated across lateral and medial angular sweeps whicheach begin and end at similar angular end points with respect to thechosen reference axis (e.g., home axis A_(H)).

As best seen in FIGS. 3A and 5, one aspect of the asymmetric peripheryof baseplate 12 arises from R1R_(X) being substantially larger thanR2L_(X). Table 1, below, also includes a comparison of radii R1R_(X) andR2L_(X) across nine exemplary component sizes, demonstrating thatdifference Δ−12RL between radius R1R_(X) and radius R2L_(X) may be aslittle as 48%, 76% or 78%, and may be as much as 102%, 103% or 149%. Itis contemplated that radius R1R_(X) may be larger than radius R2L_(X) byany percentage value within any range defined by the listed values.

TABLE 1 Comparisons of Values of Respective Medial and Lateral AnteriorCorner Radii Δ-12RL SIZE R1R vs. R2L 1 / A 103.0% 2 / B 149.2% 3 / C82.4% 4 / D 74.6% 5 / E 90.9% 6 / F 78.6% 7 / G 102.2% 8 / H 86.5% 9 / J48.1% AVG 90.6% All Δ values are expressed as the difference between agiven pair of radii, expressed as a percentage of the smaller of the tworadii

Stated another way, the smaller R2L_(X) makes a sharper turn, therebyimparting a relatively more “boxy” appearance to the anterior corner oflateral compartment 20, while the relatively larger radius R1R_(X) makesa more gradual turn that imparts a more “rounded” appearance to theanterior corner of medial compartment 22. In the exemplary nine sizesillustrated in FIG. 2A and shown in Table 1, an average disparitybetween the lateral and medial anterior corner radii R2L_(X) and R1R_(X)is greater than 90%. In some sizes of periphery 200 _(X), theanterior-medial “corner” making the more gradual turn may also includesmedial edge arc 222.

As described in detail below, this “rounded-medial/boxy-lateral”asymmetry of the anterior corners of tibial plateau facilitates andencourages proper rotational orientation and positioning of baseplate 12upon tibia T upon implantation by allowing periphery 200 to closelymatch the periphery of a typical resected tibia T (FIG. 5), while alsomaximizing the surface area of proximal surface 34 of tibial plateau toallow for use of a tibial bearing component 14 with a concomitantlylarge proximal surface area.

As noted above, the small-radius “corner” defined by angle 2L may beconsidered to have a similar angular sweep as a large-radius “corner”defined by angles 1R, 2R (or a combination of portions thereof) forpurposes of comparing the two radii. Given this comparable angularsweep, another measure of the asymmetry defined by the medial andlateral anterior corners is the arc length of the corners. Moreparticularly, because medial radii R1R_(X) and R2R_(X) are larger thanlateral radius R2L_(X) (as described above), it follows that the medialcorner has a larger arc length as compared to the lateral corner arclength for a given angular sweep.

Moreover, while the peripheries of lateral and medial compartments 20,22 are shown as being generally rounded and therefore definingrespective radii, it is contemplated that an asymmetric periphery inaccordance with the present disclosure need not define a radius per se,but rather could include one or more straight line segments which, onthe whole, define asymmetric corner edge lengths in the medial andlateral compartments. Referring to FIGS. 3B, for example, it iscontemplated that an alternative anterior lateral corner 210′ could becomprised of three line segments 210A, 210B, 210C which cooperate tospan angular extent 2L. Similarly, an alternative anterior medial corner220′ could be comprised of three line segments 220A, 220B, 220C whichcooperate to span angular extent 1R. Any of the other arcs which defineperiphery 200 could be similarly configured as one or more linesegments. In the variant illustrated by FIGS. 3B and 3C, the differencebetween corner radii would not be an appropriate measure of asymmetrybecause the straight line segments would not define radii. Asymmetry ofthe medial and lateral anterior corners would instead be quantified bycomparison of the respective lengths of the medial and lateral corneredges across comparable medial and lateral angular extents.

Yet another way to quantify the asymmetry of the anterior corner arcs(i.e., anterior-lateral corner arc 210 and anterior-medial corner arc220) is to compare the distance of the lateral and medial radius centersC2L and C1R respectively, from anterior edge 202 and/or mediolateralaxis A_(ML) (FIG. 3A). In the boxy anterior-lateral corner, centerC2L_(X) of radius R2L_(X) is anterior of mediolateral axis A_(ML) andrelatively close to anterior edge 202. For the rounded, anterior-medialcorner, centers C1R_(X) and C2R_(X) of radii R1R_(X) and R2R_(X),respectively, are posterior of mediolateral axis A_(ML) and relativelyfar from anterior edge 202.

Another metric for quantifying the “boxy vs. rounded” asymmetry ofperiphery 200 is a comparison between ratios of adjacent radii. In themore boxy lateral compartment 20, pairs of adjacent radii define largeratios because the large edge radii (i.e., of lateral anterior edge arc208, lateral edge arc 212 and lateral posterior edge arc 216) are muchlarger than the adjacent corner radii (i.e., of anterior-lateral cornerarc 210 and posterior-lateral corner arc 214). On the other hand, in themore rounded medial compartment 22, pairs of adjacent radii define smallratios (i.e., nearly 1:1) because the radii of the medial arcs (i.e.,anterior-medial corner arc 220, medial edge arc 222 and posterior-medialcorner arc 224) are of similar magnitude.

In the illustrated embodiment of FIG. 3A, lateral edge arc 212 isconsidered an “edge” because arc 212 defines tangent 212A which issubstantially perpendicular to anterior edge 202. Just as a “corner” maybe considered to be the portion of periphery 200 which makes atransition from anterior or posterior to medial or lateral, an edge isthat portion of periphery 200 which encompasses the anterior, posterior,medial or lateral terminus of periphery 200.

Similarly, medial edge arc 222 defines tangent 222A which is alsosubstantially perpendicular to anterior edge 202. The medial “edge” ofperiphery 200 may be part of the same arc that extends around theanterior-medial corner and/or the anterior-lateral corner, as the medialarcs are similar. Indeed, as noted herein, medial compartment 22 mayhave a single arc which extends from anterior edge 202 to medialposterior edge 206.

Table 2 shows a comparison between adjacent-radii ratios for lateral andmedial compartments 20 and 22. For each adjacent pair of radii, thedifference between the radii magnitudes are expressed as a percentage ofthe smaller radius of the pair, as noted above.

TABLE 2 Comparisons of Values of Respective Pairs of BaseplatePeripheral Radii Δ-12R Δ-23R Δ-12L Δ-23L Δ-34L Δ-45L R1R vs. R2R vs. R1Lvs. R2L vs. R3L vs. R4L vs. SIZE R2R R3R R2L R3L R4L R5L 1/A 18.3% 58.6%337.3% 141.8% 323.5% 194.1% 2/B 49.0% 62.0% 254.1% 96.7% 361.5% 315.4%3/C 24.0% 48.8% 247.1% 58.8% 203.4% 214.6% 4/D 44.2% 34.4% 207.0% 59.2%213.9% 244.4% 5/E 23.3% 57.9% 151.5% 80.6% 250.0% 250.0% 6/F 46.5% 37.6%122.6% 42.9% 222.6% 260.2% 7/G 25.3% 38.9% 110.8% 64.5% 264.3% 176.2%8/H 73.6% 21.3% 109.0% 80.9% 198.1% 142.6% 9/J 21.9% 61.2% 70.4% 68.5%264.0% 172.0% AVG 36.2% 46.7% 178.9% 77.1% 255.7% 218.8% All Δ valuesare expressed as the difference between a given pair of radii, expressedas a percentage of the smaller of the two radii

As illustrated in Table 2, the “boxy” periphery of lateral compartment20 gives rise to disparity values Δ−12L, Δ−23L, Δ−34L and Δ−45L that areat least 42%, 48% or 59%, and as great as 323%, 337% or 362%. It iscontemplated that the disparity between a pair of adjacent radii in theboxy periphery of lateral compartment 20 may be any percentage valuewithin any range defined by any of the listed values. It is alsocontemplated that the lateral disparity values may be substantiallyhigher, as required or desired for a particular application.

Meanwhile, the “rounded” periphery of medial compartment 22 gives riseto disparity values Δ−12R and Δ−23R that are as small as 21%, 23% or25%, and no greater than 61%, 62% or 74%. It is contemplated that thedisparity between a pair of adjacent radii in the rounded periphery ofmedial compartment 22 may be any value within any range defined by anyof the listed values. It is also contemplated that the medial disparityvalues may be less than 21%, and as little as zero %, as required ordesired for a particular application.

Moreover, the boxy shape of lateral compartment 20 and rounded shape ofmedial compartment 22 is also demonstrated by the number of arcs used todefine the portion of periphery 200 in lateral and medial compartments20, 22. In lateral compartment 20, five arcs (i.e., arcs 208, 210, 212,204, 216) are used to define the lateral periphery, which is indicativeof anterior, lateral and posterior “sides” of a box joined by therelatively sharp transitions of corner arcs 210, 214. On the other hand,medial compartment 22 uses only three radii (i.e., 220, 222, 224),leaving no clear definition of any box “sides” or other transitions.Indeed, it is contemplated that medial compartment 22 could joinanterior edge 202 to medial posterior edge 206 by a single radius withinthe scope of the present disclosure.

b. Surface Area of Medial and Lateral Baseplate Compartments

Referring still to FIG. 3A, yet another characterization of theasymmetry of periphery 200 arises from disparities in surface area forlateral and medial compartments 20, 22. For purposes of the presentdisclosure, surface area of lateral compartment SAL is that areacontained within periphery 200, and on the lateral side of home axisA_(H). Similarly, the surface area of medial compartment 22 is that areacontained within periphery 200, and on the medial side of home axisA_(H).

In an exemplary embodiment, lateral surface area SAL_(X) may be aslittle as 844 mm² or may be as much as 1892 mm², or may be any areawithin the range defined by the foregoing values. In an exemplaryembodiment, medial surface area SAM_(X) may be as little as 899 mm² ormay be as much as 2140 mm², or may be any area within the range definedby the foregoing values.

Surfaces areas SAL and SAM do not include any of the area occupied byPCL cutout 28, as any such area is not within periphery 200. However,the asymmetry of surface areas SAL and SAM arises primarily from thedifferences in the geometry and placement of arcs 208, 210, 212, 214,216, 220, 222, 224 rather than from any asymmetry of PCL cutout 28. Inthe illustrative embodiments of FIG. 2A, for example, PCL cutout 28 _(X)is symmetrical with respect to home axis A_(H), but extends furtherposteriorly in medial compartment 22.

Thus, it is contemplated that the asymmetry of surfaces areas SAL, SAMare little changed by exclusion of the PCL cutout 28 from the areacalculation. As illustrated in FIG. 3D, PCL cutout 28 is effectivelyexcluded from calculation by extrapolating the line formed by lateralposterior edge 204 and medial posterior edge 206 inwardly to intersectwith home axis A_(H). In lateral compartment 20, such extrapolationcooperates with the lateral side of PCL cutout 28 to define lateral fillarea 80. In medial compartment 22, such extrapolation cooperates withthe medial side of PCL cutout 28 to define medial fill area 82.

In the illustrative embodiment of FIG. 3D, lateral surface area SAL_(X)′may be as little as 892 mm² or may be as much as 2066 mm², or may be anyarea within the range defined by the foregoing values. In an exemplaryembodiment, medial surface area SAM_(X)′ may be as little as 986 mm² ormay be as much as 2404 mm², or may be any area within the range definedby the foregoing values.

Tables 3 and 4 below illustrate that medial surface area SAM_(X)occupies a greater percentage of the total surface area contained withinperiphery 200 _(X), regardless of whether PCL cutout 28 is included inthe calculation. That is to say, medial fill area 82 is larger thanlateral fill area 80 by approximately the same proportion as medial andlateral surfaces areas SAM_(X), SAL_(X). In the exemplary embodiments ofFIG. 3A, medial surface area SAM_(X) occupies between 52% and 53% of thetotal surface area regardless, while lateral surface area SAM_(X)occupies the remainder. If the PCL cutout is excluded from thecalculation as shown in FIG. 3D, medial surface area SAM_(X)′ occupiesbetween 52% and 54% of the total surface area, while lateral surfacearea SAM_(X)′ occupies the remainder. With or without the PCL cutoutincluded in the calculation, it is contemplated that medial surfaceareas SAM_(X), SAM_(X)′ may occupy as little as 51% of the total surfacearea, and as much as 60% of the total surface area.

TABLE 3 Medial vs. Lateral Tibial Baseplate Surface Areas for Baseplateswith a PCL Cutout (Figs.2A and 3A) With PCL Notch Medial Surface AreaSAMx Size as % of Total Surface Area 1 / A 52% 2 / B 52% 3 / C 52% 4 / D52% 5 / E 52% 6 / F 52% 7 / G 53% 8 / H 53% 9 / J 53%

TABLE 4 Medial vs. Lateral TibialBaseplate Surface Areas for Baseplateswithout a PCL Cutout (Fig. 3D) Without PCL Notch Medial Surface AreaSAM_(X)′ Size as % of Total Surface Area 1 / A 53% 2 / B 52% 3 / C 53% 4/ D 53% 5 / E 53% 6 / F 53% 7 / G 53% 8 / H 54% 9 / J 54%

c. Anteroposterior Extent of Medial and Lateral Compartments

Still another way to characterize and quantify the asymmetry of tibialperiphery 200 is to compare the overall anteroposterior extent oflateral and medial compartments 20, 22.

Turning to FIG. 2A (which is drawn to scale, according to scales 230 and232) and FIG. 2B, lateral compartment 20 of tibial plateau 18 definesoverall lateral anteroposterior extent DAPL_(X), while medialcompartment 22 of tibial plateau 18 defines overall medialanteroposterior extent DAPM_(X), where X is an integer between 1 and 9corresponding to a particular component size as shown in FIG. 2A, asnoted above. As illustrated in Table 5 below, lateral anteroposteriorextent DAPL_(X) is less than medial anteroposterior extent DAPM_(X), forall component sizes.

This disparity in anteroposterior extent can be said to result frommedial compartment 22 extending posteriorly further than lateralcompartment 20. In the illustrative embodiment of FIG. 2B, lateralanteroposterior extent DAPL_(X) extends from anterior edge 202 tolateral posterior edge 204, while medial anteroposterior extent DAPM_(X)extends from anterior edge 202 to medial posterior edge 206. Thus, ifone takes anterior edge 202 to be the anteroposterior “zero point,” theadditional anteroposterior extent defined by medial compartment 22 isdue entirely to the further posterior position of medial posterior edge206.

As set forth in the right-hand column of Table 5, exemplary embodimentsof tibial baseplate 12 may define medial anteroposterior extent DAPM_(X)that is larger than lateral anteroposterior extent DAPL_(X) by as littleas 12.1%, 12.2% or 12.4%, and as much as 13.7%, 14.2% or 14.5%. It iscontemplated that such disparity between medial and lateralanteroposterior extents DAPM_(X), DAPL_(X) may be any percentage withinany range defined by the listed values of Table 5. Advantageously, theparticular asymmetric arrangement of tibial baseplate 12 with respect toanteroposterior extent of lateral and medial compartments 20, 22facilitates substantially complete coverage of tibia T, withoutoverhanging the edge of tibia T, in a wide variety of patients.

TABLE 5 Overall A/P and M/L Dimensions for Tibial Baseplates (Figs. 2Aand 2B) Growth in NP Medial Growth in NP Lateral Additional NP Dimension(DAPM), Dimension (DAPL), Extent of DAPM from next-smaller size, fromnext-smaller size, vs. DAPL, % of Size (X) mm mm DAPL 1 / A — — 14.5% 2/ B 2.3 2.13 14.2% 3 / C 2.4 2.25 13.7% 4 / D 2.3 2.27 13.1% 5 / E 3 2.812.7% 6 / F 3.1 2.85 12.4% 7 / G 3.2 2.81 12.5% 8 / H 3.3 3.11 12.2% 9 /J 3.73 3.34 12.1%

For example, in an exemplary family of prosthesis sizes, at least 60%and as much as 90% coverage of the resected proximal surface is providedby tibial plateau 18 of tibial baseplate 12 when rotation is limited to+/−5 degrees from home axis A_(H). In a majority of all patients, suchcoverage is between 75-85%. Coverage of up to 100% may be achievedwithin the scope of the present disclosure, such as by fully extendingthe posterior-medial and anterior-lateral coverage of tibial plateau(which intentionally leave gaps between tibial plateau 18 and theperiphery of tibia T as noted herein).

The additional posteromedial material of tibial plateau 18 includeschamfer 32, described in detail below with respect to the assembly oftibial baseplate 12 to tibial bearing component 14. Chamfer 32 is formedin peripheral wall 25, such that chamfer 32 forms angle a (FIG. 8) withthe distal or bone-contacting surface 35 of tibial plateau 18. In theillustrated embodiment, chamfer 32 defines a substantially linearsagittal cross-sectional profile, with angle a between about 35 degreesand about 55 degrees. In addition, it is contemplated that chamfer 32may have an arcuate profile in a sagittal, coronal and/or transverseplane, and may include convex or concave curvature as required ordesired for a particular application.

2. Progressive Peripheral Growth Between Implant Sizes

In addition to the asymmetry of each individual size/embodiment oftibial baseplate 12, described in detail above, the present disclosurealso provides asymmetry in the way periphery 200 grows from one size tothe next. Advantageously, this asymmetric peripheral growth accommodatesobserved growth trends in tibias T of differently-sized patients, whilealso preserving the optimal fit and coverage provided by baseplate 12,and offering the other advantages of designs in accordance with thepresent disclosure as described herein.

In symmetrical peripheral growth, a larger size of baseplate is ascaled-up version of a smaller size and vice-versa. In the presentasymmetrical peripheral growth, by contrast, certain parameters oftibial baseplate 12 grow faster than others as the overall size of thebaseplate gets larger (i.e., from smallest size 1/A through largest size9/J). Thus, differently-sized components made in accordance with thepresent disclosure are not proportional to one another in all respects,in that a larger tibial prosthesis is not proportionally larger than asmaller tibial prosthesis in all aspects.

Referring now to FIG. 2B, periphery 200 _(X) defines centroid C_(X),which is medially biased with respect to home axis A_(H) owing to medialsurface area SAM being larger than lateral surface area SAL (asdescribed in detail above). Posterior-medial distance DMP_(X) extendsfrom centroid C_(X) toward the posterior-medial “corner” of periphery200 _(X) (i.e., toward posterior-medial corner arc 224, shown in FIG. 3Aand described above) at an angle of 130 counter-clockwise degrees fromhome axis A_(H). Similarly, posterior-lateral distance DLP_(X) extendsfrom centroid C_(X) toward the posterior-lateral “corner” of periphery200 _(X) (i.e., toward posterior-lateral corner arc 214, shown in FIG.3A and described above) at an angle of 120 clockwise degrees from homeaxis A_(H). The posterior-lateral and posterior-medial corners aredefined in a similar fashion as the anterior-lateral and anterior-medialcorners, described in detail above. Moreover, while the asymmetricposterior-medial and posterior lateral growth among consecutive sizes isdescribed below with respect to distances DLP_(X), DMP_(X), such growthoccurs in the entire area occupied by the posterior-medial andposterior-lateral corners.

As illustrated in FIG. 2A and shown in Table 6 below, lateral- andmedial-posterior distances DLP_(X), DMP_(X) do not grow linearly assmallest size 1/A progresses among consecutive sizes to eventually reachlargest size 9/J. Rather, lateral- and medial-posterior distancesDLP_(X), DMP_(X) exhibit an increase in the magnitude of growth as thesizes progress consecutively from size 1/A to size 9/J. This non-linear,asymmetric growth is illustrated in the graphs of FIGS. 2C and 2D and inTable 6 below.

TABLE 6 Growth of the Posterior-Medial and Posterior-Lateral Corners ofBaseplate Periphery (Figs. 2A and 2B) Growth in medial-posterior Growthin lateral-posterior distance DMP_(X) from distance (DLP_(X)) fromcentroid (C_(X)),compared to centroid (C_(X)), compared to Size (X)next-smaller size, mm next-smaller size, mm 1 — — 2 2.42 2.48 3 2.56 2.84 2.76 2.55 5 2.86 3.26 6 3.71 2.64 7 3.28 2.83 8 3.52 2.28 9 3.76 3.29

In FIG. 2C, the amount of growth in DMP_(X) is plotted against size no.X. As illustrated, the family of tibial baseplates 12 illustrated inFIG. 2A exhibit a steadily increasing growth in DMP_(X), with nearly 20%average increase in growth from one size to the next consecutive size(as represented by the slope of the linear trend line having equationy=0.1975x+2.0225).

In FIG. 2D, the amount of growth in DLP_(X) is plotted against size no.X, and illustrates a smaller, but still positive growth increase acrossbaseplate sizes. More specifically, the family of tibial baseplates 12illustrated in FIG. 2A exhibit a nearly 4% average increase in growthfrom one size to the next consecutive size (as represented by the slopeof the linear trend line having equation y=0.0392x+2.5508).

As used herein, a “family” of prostheses refers to a set or kit ofprostheses sharing common geometrical and/or performancecharacteristics. For example, the family of nine tibial baseplates whoseperipheries 200 _(X) are shown in FIG. 2A share a common asymmetry asdescribed herein, such that each tibial baseplate is adapted to providesubstantial tibial coverage, facilitate proper implant rotation andavoid impingement with various soft tissues of the knee. Typically, afamily of prostheses includes a plurality of differently-sizedcomponents, with consecutively larger/smaller components sized toaccommodate a variety of differently-sized bones. In the exemplaryembodiments of the present disclosure, a size “1” or “A” prosthesis isthe smallest prosthesis of the family, a size “9” or “J” prosthesis isthe largest prosthesis of the family, and each of the intermediate sizes“2” or “B” through “8” or “H” are consecutively larger sizes.

Advantageously, in the family or kit of prosthesis peripheries shown inFIG. 2A, each tibial baseplate 12 (FIG. 1A) having periphery 200 _(X)provides a close match to a particular subset of patient tibias T havinga unique size and shape. Particular features of periphery 200 _(X) havebeen designed with non-linear growth which is calculated to provide theclosest possible fit for the largest number of particular naturalgeometries found in anatomic tibias T, as described in detail herein.This close fit allows for maximum coverage of the resected proximaltibial periphery 200 _(X), by accommodating the non-linear changes whichmay occur across anatomic tibial periphery sizes. Lateral- andmedial-posterior distances DLP_(X), DMP_(X) are exemplary non-lineargrowth parameters found in a family of tibial baseplates 12, and arereflective of non-linear growth in mediolateral extent DML_(X) andanteroposterior extents DAPM_(X) and DAPL_(X) across the various sizes.

3. PCL Cutout Aligned with Home Axis and Associated Technique

In the illustrated embodiment, tibial plateau 18 includes PCL cutout 28disposed between compartments 20, 22, as described above. PCL cutoutleaves PCL attachment point C_(P) accessible, thereby allowing the PCLto pass therethrough during and after implantation of tibial baseplate12. Tibial bearing component 14 (FIG. 5) may similarly include cutout30.

Thus, the illustrated embodiment of tibial prosthesis 10 is adapted fora cruciate retaining (CR) surgical procedure, in which the posteriorcruciate ligament is not resected during implantation of tibialprosthesis 10. Further, as noted above, home axis A_(H) includesreference to PCL attachment point C_(P) when tibial baseplate 12 ismounted upon tibia T. In order to facilitate alignment of home axisA_(H) with respect to tibial baseplate 12 and tibia T, alignment indicia70A, 70P (FIGS. 4A and 4B) may be marked on proximal surface 34 and/orperipheral wall 25. When tibial baseplate 12 is implanted (as describedbelow), anterior alignment indicia 70A (FIGS. 4A and 4B) is aligned withanterior point C_(A) at the “medial third” of the anterior tibialtubercle T, and posterior alignment indicia 70P is aligned with thenatural PCL attachment point C_(P) of tibia T.

However, it is contemplated that a prosthesis in accordance with thepresent disclosure may be made for a design in which the posteriorcruciate ligament is resected during surgery, such as “posteriorstabilized” (PS) or “ultra congruent” (UC) designs. The PS and UCdesigns may exclude PCL cutout 30 in bearing component 14, therebyobviating the need for PCL cutout 28 in tibial baseplate 12. Continuousmaterial may instead occupy cutout 28 (as schematically shown in FIG.3D). Moreover, it is contemplated that PCL cutouts 28, 30 may have anyshape and/or size within the scope of the present disclosure. Forexample, PCL cutouts 28, 30 may be asymmetrical with respect to ananteroposterior axis. For purposes of the present disclosure “bisecting”an asymmetric PCL cutout with an anteroposterior axis refers to dividingsuch cutout into two equal areas for a given anteroposterior section ofthe anteroposterior axis

4. Tibial Bearing Component and Deep Flexion Enablement

Turning again to FIG. 1A, tibial bearing component 14 includes lateralportion 39, medial portion 41, inferior surface 36 adapted to couple totibial baseplate 12, and superior surface 38 adapted to articulate withcondyles of a femoral component (such as femoral component 60 shown inFIG. 8 and described in detail below). Superior surface 38 includeslateral articular surface 40 in lateral portion 39 and medial articularsurface 42 in medial portion 41, with eminence 44 (FIG. 5) disposedbetween articular surfaces 40. 42. Referring to FIG. 5, eminence 44generally corresponds in shape and size with a natural tibial eminenceof tibia T prior to resection.

Referring now to FIG. 1A, tibial plateau 18 of tibial baseplate 12further includes a distal or bone contacting surface 35 and an opposingproximal or superior surface 34, with superior surface 34 having raisedperimeter 24 and locking mechanism 26 formed between lateral and medialcompartments 20, 22. Raised perimeter 24 and locking mechanism 26cooperate to retain tibial bearing component 14 upon tibial baseplate12, as described in detail below. Exemplary baseplate locking mechanismsare described in U.S. provisional patent application Ser. Nos.61/367,374 and 61/367,375, both entitled TIBIAL PROSTHESIS incorporatedby reference above in paragraph [0001].

Inferior surface 36 of tibial bearing component 14 includes recess 46 atthe periphery thereof and a tibial bearing locking mechanism (not shown)disposed between lateral and medial articular surfaces 40, 42. Exemplarybearing component locking mechanisms are disclosed in U.S. provisionalpatent application Ser. Nos. 61/367,374 and 61/367,375, both entitledTIBIAL PROSTHESIS. Recess 46 is sized and positioned to correspond withraised perimeter 24 of tibial plateau 18, and the tibial bearing lockingmechanism cooperates with locking mechanism 26 of tibial plateau 18 tofix tibial bearing component 14 to tibial baseplate 12 in a desiredposition and orientation as described in detail below. However, it iscontemplated that tibial bearing component 14 may be affixed tobaseplate 12 by any suitable mechanism or method within the scope of thepresent disclosure, such as by adhesive, dovetail tongue/groovearrangements, snap-action mechanisms, and the like.

As best seen in FIGS. 1B, 5 and 8, the outer periphery of tibial bearingcomponent 14 generally corresponds with the outer periphery of tibialplateau 18, except for the posteromedial extent of plateau 18 ascompared with tibial bearing component 14. The anterolateral “corner” oftibial bearing component 14 defines radius R₃ (FIG. 5) having agenerally common center with radius R2L of baseplate 12 in a transverseplane, i.e., radii R2L and R₃ are substantially coincident in a planview. Similarly, the anteromedial “corner” of tibial bearing component14 defines radius R₄ having a generally common center with radius R1R ofbaseplate 12 in a transverse plane, i.e., radii R1R and R₄ aresubstantially coincident in a plan view.

R₃ defines a slightly smaller radial length as compared to R2L, and R₄defines a slightly smaller radial length as compared to R1R, such thatthe anterior portion of perimeter wall 54 of tibial bearing component 14is set back from the anterior portion of peripheral wall 25 (i.e., fromanterior edge 202 and adjacent arcs, as described above) of tibialbaseplate 12. As with the above-described comparison between radii R2Land R1R, anteromedial radius R₄ is substantially larger thananterolateral radius R₃.

Given that medial portion 41 of tibial bearing component 14 has a lesseranteroposterior extent compared to medial compartment 22 of tibialplateau 18, medial portion 41 must be biased anteriorly in order for theanterior-medial “corners” of tibial bearing component 14 and tibialplateau 18 to coincide as shown in FIG. 5. In view of this anteriorbias, it may be said that tibial bearing component 14 is asymmetricallyoriented upon tibial plateau 18. More particularly, although lateralarticular surface 40 is generally centered with respect to lateralcompartment 20 of tibial plateau 18, medial articular surface 42 isanteriorly biased with respect to medial compartment 22 of tibialplateau 18 in order to leave chamfer 32 exposed at the posterior-lateralcorner. This asymmetric mounting of tibial bearing component 14 upontibial plateau 18 ensures a desired articular interaction between tibialprosthesis 10 and femoral component 60, as described in detail below.

Tibial plateau 18 of tibial baseplate 12 deviates from the periphery oftibial bearing component 14 in the posteromedial portion of eachcomponent, leaving medial portion 41 incongruent with medial compartment22 of tibial baseplate 12. More particularly, tibial plateau 18 extendsposteromedially to substantially cover the proximal resected surface oftibia T, as shown in FIG. 5 and described in above, while tibial bearingcomponent 14 does not extend posteromedially beyond the superiorterminus of chamfer 32 (i.e., tibial bearing component 14 does not“overhang” chamfer 32). In addition, tibial bearing component 14includes chamfer 50 formed in peripheral wall 54, with chamfer 50 havinga profile and geometrical arrangement corresponding with chamfer 32 oftibial plateau 18. More particularly, when tibial bearing component 14is assembled to tibial baseplate 12 as shown in FIGS. 1B and 8, theanterior orientation or “bias” of the medial portion of tibial bearingcomponent 14 (as described above) aligns chamfers 32, 50, which in turncooperate to create a substantially continuous chamfer extending fromtibia T to medial articular surface 42. Referring to FIG. 8, chamfers32, 50 further cooperate to define void 52 formed between femur F andtibial plateau 18 when tibial prosthesis 10 is in a deep flexionorientation. In the illustrated embodiment of FIG. 8, the deep flexionorientation is defined by angle β between anatomic tibia axis A_(T) andanatomic femoral axis A_(F) of up to about 25 degrees to about 40degrees, for example (i.e., about 140 degrees to 155 degrees of flexionor more).

Advantageously, void 52 cooperates with the “pulled back” or incongruentposterior medial edge 206 and posterior medial corner 224, as comparedto a typical tibial periphery (described above), to allow the deepflexion orientation to be achieved without impingement of femoralcomponent 60 and/or femur F upon tibial plateau 18 and/or tibial bearingcomponent 14. Soft tissues in the region of void 52 are therefore alsoaccommodated with little or no impingement on the surroundingcomponents.

In addition, the relatively large size of tibial plateau 18 (covering alarge proportion of the resected proximal surface of tibia T) alsoallows tibial bearing component 14 to be relatively large, so thattibial bearing component 14 provides sufficient non-articular surfacearea at chamfers 32, 50 and around the periphery of lateral and medialarticular surfaces 40, 42 to allow relatively large-radius, roundedtransitions between articular surfaces 40, 42 and peripheral wall 54 oftibial bearing component 14. These gradual, large-radius transitionsprevent undue friction between tibial prosthesis 10 and any surroundingsoft tissues which may remain in place after implantation of theprosthesis, such as the iliotibial (IT) band.

In certain ranges of prosthesis articulation, for example, the humaniliotibial (IT) band may touch the anterolateral “corner”, i.e., theportion of tibial bearing component 14 having radius R₃. Because theanterolateral extent of tibial bearing component 14 follows theanterolateral extent of tibial plateau 18 (as described above), thetransition between lateral articular surface 40 and peripheral wall 54at the point of contact between an IT band and tibial bearing component14 can have a relatively large convex portion while still leavingsufficient concave space for articular surface 40. This large convexportion results in a large contact area if the IT band does contacttibial bearing component 14, which in turn results in relatively lowpressures on the IT band. Further, the anterolateral “pull back” orincongruence between the anterior-lateral corner arc 210 of periphery200 and a typical tibial periphery, described in detail above, allowsthe corresponding anterior-lateral corner of bearing component 14 tomaintain separation from the IT band through a wide range of flexion,and low contact pressures where contact does occur.

However, to any such contact between the IT band and tibial bearingcomponent 14 may be avoided or minimized by designing periphery 200 suchthat anterior-lateral corner arc 210 and/or lateral edge arc 212 isbrought away from the expected periphery of a typical tibia T (ascalculated from anatomical data, described above). This extra spacingdesigned into periphery 200 provides extra clearance for the iliotibialband. In addition, this extra clearance assures that the substantialproportion of prospective patients lacking Gerdy's tubercle, which is aneminence located at the anterior-lateral portion of tibia T, will notexperience any “overhang” of tibial plateau 18 beyond the anatomicperiphery of resected tibia T.

Thus, generally speaking, tibial prosthesis 10 can be considered “softtissue friendly” because the edges of tibial bearing component 14 andtibial plateau 18, including chamfers 32, 50, are smooth and rounded, sothat any soft tissue coming into contact with these edges will be lesslikely to chafe or abrade.

Advantageously, the relatively large inferior/distal surface area oftibial plateau 18 facilitates a large amount of bone ingrowth where boneingrowth material is provided in tibial baseplate 12. For example,baseplate 12 may also be constructed of, or may be coated with, a highlyporous biomaterial. A highly porous biomaterial is useful as a bonesubstitute and as cell and tissue receptive material. A highly porousbiomaterial may have a porosity as low as 55%, 65%, or 75% or as high as80%, 85%, or 90%. An example of such a material is produced usingTrabecular Metal™ Technology generally available from Zimmer, Inc., ofWarsaw, Indiana. Trabecular Metal™ is a trademark of Zimmer, Inc. Such amaterial may be formed from a reticulated vitreous carbon foam substratewhich is infiltrated and coated with a biocompatible metal, such astantalum, by a chemical vapor deposition (“CVD”) process in the mannerdisclosed in detail in U.S. Pat. No. 5,282,861 to Kaplan, the entiredisclosure of which is expressly incorporated herein by reference. Inaddition to tantalum, other metals such as niobium, or alloys oftantalum and niobium with one another or with other metals may also beused.

Generally, the porous tantalum structure includes a large plurality ofligaments defining open spaces therebetween, with each ligamentgenerally including a carbon core covered by a thin film of metal suchas tantalum, for example. The open spaces between the ligaments form amatrix of continuous channels having no dead ends, such that growth ofcancellous bone through the porous tantalum structure is uninhibited.The porous tantalum may include up to 75%, 85%, or more void spacetherein. Thus, porous tantalum is a lightweight, strong porous structurewhich is substantially uniform and consistent in composition, andclosely resembles the structure of natural cancellous bone, therebyproviding a matrix into which cancellous bone may grow to providefixation of implant [#] to the patient's bone.

The porous tantalum structure may be made in a variety of densities inorder to selectively tailor the structure for particular applications.In particular, as discussed in the above-incorporated U.S. Pat. No.5,282,861, the porous tantalum may be fabricated to virtually anydesired porosity and pore size, and can thus be matched with thesurrounding natural bone in order to provide an improved matrix for boneingrowth and mineralization.

5. Trial Tibial Components

Tibial prosthesis 10 may be provided in a variety of sizes andconfigurations to accommodate different bone sizes and geometries. Thechoice of one particular size may be planned preoperatively such asthrough preoperative imaging and other planning procedures.Alternatively, an implant size may be chosen, or a previous size choicemodified, intraoperatively. To facilitate proper intraoperativeselection of a particular size for tibial prosthesis 10 from among thefamily of sizes shown in FIG. 2A, and to promote proper orientation ofthe chosen prosthesis 10, tibial prosthesis 10 may be part of a kitincluding one or more template or “sizing” components.

Referring now to FIGS. 6 and 7, trial prosthesis 100 may be temporarilycoupled to tibia T for intraoperative sizing evaluation of tibialprosthesis 10 and initial steps in the implantation of tibial prosthesis10. Trial prosthesis 100 is one of a set of trial prostheses provided asa kit, with each trial prosthesis having a different size andgeometrical configuration. Each trial prosthesis in the set of trialprostheses corresponds to a permanent prosthesis 10, such as sizes1/A-9/J of tibial baseplate 12 as described above.

For example, as shown in FIG. 6, trial prosthesis 100 defines superiorsurface 112 generally corresponding in size and shape to proximalsurface 34 of tibial plateau 18, and including lateral portion 102 andmedial portion 104. Superior surface 112 is asymmetrical about home axisA_(H), with lateral portion 102 having a generally shorter overallanteroposterior extent as compared to medial portion 104 (which includesvoid indicator 106, discussed below). In addition, the anterolateral“corner” of lateral portion 102 defines radius R2L, which is identicalto radius R2L of tibial plateau 18, while the anteromedial “corner” ofmedial portion 104 defines radius R1R, which is identical to radius R1Rof tibial plateau 18 and greater than radius R2L.

Moreover, perimeter wall 114 of trial prosthesis 100 is substantiallyidentical to peripheral wall 25 of tibial plateau 18, and thereforedefines periphery 200 with the same features and shapes of perimeter 200described above with respect to tibial plateau 18. Thus, trialprosthesis 100 is asymmetrical about home axis A_(H) in a similar mannerto tibial plateau 18 of tibial baseplate 12, with the nature of thisasymmetry changing across the various other sizes of tibial prosthesisprovided in the kit including trial prosthesis 100.

In an alternative embodiment, a trial prosthesis may be provided whichextends completely to the posterior-medial edge of the natural tibialresection periphery. Thus, such a trial would substantially completelycover the resected tibial surface, thereby aiding in determination of aproper rotational orientation of the trial (and, therefore, of the finaltibial baseplate 12). In this alternative embodiment, the trialprosthesis lacks the posterior-medial “pull back” of tibial plateau 18,described above.

Trial prosthesis 100 includes void indicator 106 disposed at theposterior portion of medial portion 104, consuming a given posteromedialarea of superior surface 34 and peripheral wall 25. Void indicator 106indicates where void 52 (discussed above) will be located with respectto tibia T after implantation of tibial prosthesis 10. Void indicator106 facilitates proper rotational and spatial orientation of trialprosthesis 100 on the resected proximal surface of tibia T by allowing asurgeon to visually match tibial bearing component 14 with trialprosthesis 100, as described in detail below. In the illustratedembodiment, void indicator 106 is an area of visual and/or tactilecontrast with the remainder of tibial plateau 18. This contrast mayinclude, for example, a contrasting color, texture, surface finish, orthe like, or may be formed by a geometric discrepancy such as a step orlip, for example.

Referring specifically to FIG. 6, trial prosthesis 100 further includesa plurality of peg hole locators 108 corresponding to the properlocation for peg holes in tibia T to receive pegs (not shown) extendinginferiorly from tibial plateau 18 of tibial baseplate 12.Advantageously, peg hole locators 108 allow a surgeon to demarcate theproper center for peg holes in tibia T once the proper size andorientation for trial prosthesis 100 has been found, as discussed indetail below. Alternatively, peg hole locators 108 may be used as drillguides to drill appropriately positioned peg holes while trialprosthesis is still positioned on tibia T.

6. Tibial Prosthesis Implantation

In use, a surgeon first performs a resection of tibia T usingconventional procedures and tools, as are well-known in the art. In anexemplary embodiment, a surgeon will resect the proximal tibia to leavea planar surface prepared for receipt of a tibial baseplate. This planarsurface may define a tibial slope, which is chosen by the surgeon. Forexample, the surgeon may wish to perform a resection resulting inpositive tibial slope in which the resected tibial surface slopesproximally from posterior to anterior (i.e., the resected surface runs“uphill” from posterior to anterior). Alternatively, the surgeon mayinstead opt for negative tibial slope in which the resected tibialsurface slopes distally from posterior to anterior (i.e., the resectedsurface runs “downhill” from posterior to anterior). Varus or valgusslopes may also be employed, in which the resected surface slopesproximally or distally from medial to lateral. The choice of a tibialand/or varus/valgus slope, and the amount or angle of such slopes, maydepend upon a variety of factors including correction of deformities,mimicry of the native/preoperative tibial slope, and the like.

In an exemplary embodiment, keel 16 (FIG. 4B) defines a 5-degree,anteriorly-extending angle with respect to bone-contacting surface 35 oftibial plateau 18. Tibial baseplate 12 is appropriate for use with apositive tibial slope of as little as zero degrees and as much as 9degrees, and with a varus or valgus slope of up to 3 degrees. However,it is contemplated that a tibial baseplate made in accordance with thepresent disclosure may be used with any combination of tibial and/orvarus/valgus slopes, such as by changing the angular configuration ofthe keel with respect to the bone-contacting surface.

With a properly resected proximal tibial surface, the surgeon selectstrial prosthesis 100 from a kit of trial prostheses, with eachprosthesis in the kit having a different size and geometricalconfiguration (as discussed above). Trial prosthesis 100 is overlaid onthe resected surface of tibia T. If trial prosthesis 100 isappropriately sized, a small buffer zone 110 of exposed bone of resectedtibia T will be visible around the periphery of trial prosthesis 100.Buffer 110 is large enough to allow a surgeon to rotate and/orreposition trial prosthesis 100 within a small range, thereby offeringthe surgeon some flexibility in the final positioning and kinematicprofile of tibial prosthesis 10. However, buffer 110 is small enough toprevent trial prosthesis 100 from being rotated or moved to an improperlocation or orientation, or from being implanted in such as way as toproduce excessive overhang of the edge of trial prosthesis 100 past theperiphery of the resected tibial surface. In one exemplary embodiment,for example, trial prosthesis may be rotated from a centered orientationby up to +/−5 degrees (i.e., in either direction), though it iscontemplated that such rotation may be as much as +/−10 degrees or +/−15degrees.

To aid in rotational orientation, trial prosthesis may include anteriorand posterior alignment indicia 70A, 70P, which are the same marks inthe same location as indicia 70A, 70P provided on tibial plateau 18 asdescribed above. The surgeon can align indicia 70A with anterior pointC_(A) and indicia 70P with PCL attachment point C_(P), in similarfashion as described above, to ensure the anatomical and component homeaxes A_(H) are properly aligned. Alternatively, a surgeon may useindicia 70A, 70P to indicate a desired deviance from alignment with homeaxis A_(H). As noted above, deviation of up to 5 degrees is envisionedwith the exemplary embodiments described herein. A surgeon may choose toorient indicia 70A, 70P to another tibial landmark, such as the middleof the patella or the medial end of tibial tubercle B.

Thus, the large coverage of trial prosthesis 100 (and, concomitantly, oftibial plateau 18) ensures that tibial baseplate 12 will be properlypositioned and oriented on tibia T upon implantation, thereby ensuringproper kinematic interaction between tibial prosthesis 10 and femoralcomponent 60. If buffer zone 110 is either nonexistent or too large,another trial prosthesis 100 is selected from the kit and compared in asimilar fashion. This process is repeated iteratively until the surgeonhas a proper fit, such as the fit illustrated in FIGS. 6 and 7 betweentrial prosthesis 100 and tibia T.

With the proper size for trial prosthesis 100 selected and itsorientation on tibia T settled, trial prosthesis 100 is secured to tibiaT, such as by pins, screws, temporary adhesive, or any otherconventional attachment methods. Once trial prosthesis is so secured,other trial components, such as trial femoral components and trialtibial bearing components (not shown) may be positioned and used toarticulate the leg through a range of motion to ensure a desiredkinematic profile. During such articulation, void indicator 106indicates to the surgeon that any impingement of femoral component 60and/or femur F upon trial prosthesis 100 at void indicator 106 will notoccur when tibial prosthesis 10 is implanted. Once the surgeon issatisfied with the location, orientation and kinematic profile of trialprosthesis 100, peg hole locators 108 may be used to demarcate theappropriate location of peg holes in tibia T for tibial baseplate 12.Such peg holes may be drilled in tibia T with trial prosthesis 100attached, or trial prosthesis 100 may be removed prior to drilling theholes.

With tibia T prepared for receipt of tibial prosthesis 10, tibialbaseplate 12 may be provided by the surgeon (such as from a kit orsurgical inventory), and is implanted on tibia T, with pegs fitting intoholes previously identified and demarcated using peg hole locators 108of trial prosthesis 100. Tibial baseplate 12 is selected from the familyof tibial baseplates illustrated in FIG. 2A to correspond with the trialcomponent 100 chosen, which ensures that tibial plateau 18 will cover alarge proportion of the resected proximal surface of tibia T, as trialprosthesis 100 did prior to removal. Tibial baseplate is affixed totibia T by any suitable method, such as by keel 16 (FIG. 4B), adhesive,bone-ingrowth material, and the like.

With tibial baseplate 12 installed, tibial bearing component 14 may becoupled with tibial baseplate 12 to complete tibial prosthesis 10.However, once attached, tibial bearing component 14 does not fully covertibial plateau 18 of tibial baseplate 12. Rather, tibial bearingcomponent 14 leaves a posteromedial portion of tibial baseplate 12uncovered to create void 52 (as shown in FIG. 8 and discussed above).Thus, a surgeon may wish to verify that this anterior-biased,“asymmetrical” orientation of medial articular surface 42 is properprior to permanent affixation of tibial bearing component 14 to tibialbaseplate 12.

To accomplish such verification, tibial bearing component 14 is placedside-by-side with trial prosthesis 100, with inferior surface 36 oftibial bearing component 14 in contact with superior surface 112 oftrial prosthesis 100. Tibial bearing component 14 will substantiallycover superior surface 112, but will not cover void indicator 106. Putanother way, peripheral wall 54 of tibial bearing component 14 willtrace perimeter wall 114 of tibial trial prosthesis 100, excluding theposteromedial area defined by void indicator 106. If inferior surface 36of tibial bearing component 14 is a match with superior surface 112 oftrial prosthesis 100 except for void indicator 106 (which is leftuncovered by tibial bearing component 14), then tibial bearing component14 is the proper size component and may be confidently installed upontibial plateau 18 of tibial baseplate 12.

Tibial baseplate 12 may then be implanted upon the proximal surface oftibia T in accordance with accepted surgical procedures. Exemplarysurgical procedures and associated surgical instruments are disclosed in“Zimmer LPS-Flex Fixed Bearing Knee, Surgical Technique,” “NEXGENCOMPLETE KNEE SOLUTION, Surgical Technique for the CR-Flex Fixed BearingKnee” and “Zimmer NexGen Complete Knee SolutionExtramedullary/Intramedullary Tibial Resector, Surgical Technique”(collectively, the “Zimmer Surgical Techniques”), copies of which aresubmitted on even date herewith, the entire disclosures of which arehereby expressly incorporated by reference herein.

When the surgeon is satisfied that tibial bearing component 14 isproperly matched and fitted to the installed tibial baseplate 12,bearing component 14 is secured using locking mechanism 26 and thecorresponding tibial bearing locking mechanism an appropriateinstrumentation (not shown). Proper location and rotational orientationof tibial bearing component 14 upon tibial plateau 18 is ensured byraised perimeter 24 cooperating with recess 46, and locking mechanism 26cooperating with the corresponding tibial bearing locking mechanism (notshown). Such proper orientation results in medial articular surface 42being generally anteriorly disposed with respect to medial compartment22 of tibial plateau 18.

Femoral component 60 may be affixed to a distal end of femur F, ifappropriate, using any conventional methods and/or components. Exemplarysurgical procedures and instruments for such affixation are disclosed inthe Zimmer Surgical Techniques, incorporated by reference above. Femur Fand tibia T may then be articulated with respect to one another toensure that neither femur F nor femoral component 60 impinges upontibial baseplate 12 and/or tibial bearing component 14 in deep flexion,such as at a flexion angle β of 155° as shown in FIG. 8. When thesurgeon is satisfied with the location, orientation and kinematicprofile of tibial prosthesis 10, the knee replacement surgery iscompleted in accordance with conventional procedures.

While this invention has been described as having an exemplary design,the present invention can be further modified within the spirit andscope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the invention using itsgeneral principles. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains and which fallwithin the limits of the appended claims.

1. A method of implanting a tibial prosthesis on a tibia, the methodcomprising: resecting a proximal end of a tibia to create a resectedtibial surface, the tibia having an anteriorly disposed tibial tubercleand a connection point to a posterior cruciate ligament; identifying ananatomic anteroposterior axis which intersects: an anterior pointdisposed on the tubercle, the anterior point medially spaced from atubercle midpoint by an amount equal to ⅙^(th) of an overallmediolateral width of the tubercle such that the anterior point lies onthe medial third of the tubercle; and a posterior point disposed at ageometric center of an area defined by the connection point of the tibiato the posterior cruciate ligament; providing a tibial prosthesis havingalignment indicia indicating a component anteroposterior axis designedto be aligned with the anatomic anteroposterior axis; and positioningand orienting the tibial prosthesis with respect to the tibia byaligning the alignment indicia with the anatomic anteroposterior axis.2. The method of claim 1, wherein: said step of providing comprisesproviding a tibial prosthesis including: an anterior edge; and a PCLcutout area generally opposite said anterior edge and disposed between alateral compartment of the tibial prosthesis and a medial compartment ofthe tibial prosthesis, said component anteroposterior axis bisecting theanterior edge and bisecting the PCL cutout area, whereby said step ofpositioning comprises centering the anterior edge with respect to theanterior point disposed on the tubercle, and centering the PCL cutoutarea with respect to the connection point of the tibia to the posteriorcruciate ligament.
 3. The method of claim 2, wherein: the medialcompartment cooperates with the component anteroposterior axis to bounda medial surface area, the lateral compartment cooperates with thecomponent anteroposterior axis to bound a lateral surface area, themedial surface area larger than the lateral surface area.
 4. The methodof claim 3, wherein the medial surface area occupies between about 52%and about 53% of the medial and lateral surface areas combined.
 5. Themethod of claim 1, wherein: said step of providing comprises providing atibial prosthesis having a lateral compartment and a medial compartmentwhich cooperate to define a peripheral wall, the peripheral wallincluding: an anterior edge; a lateral posterior edge generally oppositethe anterior edge and forming a posterior boundary of the lateralcompartment; a medial posterior edge generally opposite the anterioredge and forming a posterior boundary of the medial compartment; alateral periphery extending from the anterior edge to the lateralposterior edge; and a medial periphery extending from the anterior edgeto the medial posterior edge; the lateral compartment is asymmetric withrespect to the medial compartment about the component anteroposterioraxis to define a component asymmetry, said step of positioningcomprising matching the component asymmetry to a corresponding asymmetryof the resected tibial surface, whereby the tibial prosthesis covers alarge proportion of the resected tibial surface.
 6. The method of claim5, wherein the component asymmetry comprises a surface area asymmetry,in which: the medial periphery cooperates with the componentanteroposterior axis to bound a medial surface area; the lateralperiphery cooperates with the component anteroposterior axis to bound alateral surface area; and the medial surface area larger than thelateral surface area, said step of positioning comprising: matching thelarger medial surface area to a corresponding medial portion of theresected tibial surface, and matching the smaller lateral surface areato a corresponding lateral portion of the resected tibial surface. 7.The method of claim 6, wherein the medial surface area occupies betweenabout 52% and about 54% of the medial and lateral surface areascombined.
 8. The method of claim 5, wherein said step of positioningincludes covering between about 60% and about 90% of the tibia to createa buffer zone on all sides between the resected tibial surface and theperipheral wall.
 9. The method of claim 5, wherein the componentasymmetry comprises an anteroposterior extent asymmetry, in which: thelateral compartment defines a lateral anteroposterior extent extendingfrom the anterior edge of the peripheral wall to the lateral posterioredge thereof; and the medial compartment defines a medialanteroposterior extent extending from the anterior edge of theperipheral wall to the medial posterior edge thereof, the medialanteroposterior extent larger than the lateral anteroposterior extent,said step of positioning comprising: matching the larger medialanteroposterior extent to a corresponding medial portion of the resectedtibial surface, and matching the smaller lateral anteroposterior extentto a corresponding lateral portion of the resected tibial surface. 10.The method of claim 5, wherein the component asymmetry comprises ananteroposterior corner asymmetry, in which: an anterior-medial corner ofthe medial periphery defines an anterior-medial corner radius, and ananterior-lateral corner of the lateral periphery defines ananterior-lateral corner radius that is substantially smaller than theanterior-medial corner radius, said step of positioning comprising:matching the smaller anterior-lateral corner radius to a correspondinglateral portion of the resected tibial surface, and matching the largeranterior-medial radius to a corresponding medial portion of the resectedtibial surface.
 11. The method of claim 5, wherein the componentasymmetry comprises a medial- lateral asymmetry, in which: the lateralperiphery defines a plurality of adjacent lateral arcs, an adjacent pairof the plurality of adjacent lateral arcs defining a first lateralradius and a second lateral radius, respectively, the first lateralradius larger than said second lateral radius by at least 100%, wherebysaid lateral periphery is relatively boxy, said step of positioningcomprising matching the boxy lateral periphery to a correspondingly boxylateral portion of the resected tibial surface.
 12. The method of claim5, wherein the component asymmetry comprises a medial-lateral asymmetry,in which: the medial periphery defines a plurality of adjacent medialarcs, a pair of the plurality of adjacent medial arcs defining a firstmedial radius and a second medial radius, respectively, the first medialradius larger than the second medial radius by less than 75%, wherebysaid medial periphery is generally rounded, said step of positioningcomprising matching the rounded medial periphery to a correspondinglyrounded medial portion of the resected tibial surface.
 13. The method ofclaim 5, wherein the tibial prosthesis comprises a tibial baseplate, themethod further comprising providing a tibial bearing componentcomprising: a lateral bearing portion which substantially corresponds tothe lateral periphery of the tibial prosthesis; and a medial bearingportion which is at least partially incongruent with the medialperiphery of the tibial prosthesis.
 14. The method of claim 13, furthercomprising mounting the tibial bearing component to the tibial baseplatesuch that the medial bearing portion is anteriorly biased with respectto the medial compartment of the tibial prosthesis.
 15. The method ofclaim 1, wherein the tibial prosthesis comprises a tibial baseplatehaving a distal surface comprising a bone contacting surface and aproximal surface comprising a tibial bearing engagement surface.
 16. Themethod of claim 15, further comprising: providing a tibial bearingcomponent; and attaching the tibial bearing component to the tibialbaseplate.
 17. The method of claim 16, further comprising: providing afemoral component; and affixing the femoral component to a femur suchthat the femoral component articulates with the tibial bearingcomponent.
 18. A method of implanting a tibial prosthesis on a tibia,the method comprising: identifying an anatomic home axis on a resectedsurface on a proximal end of a tibia, the anatomic home axis defined asa line extending from a posterior point at a geometric center of anattachment area between a posterior cruciate ligament and the tibia toan anterior point disposed on an anterior tubercle of the tibia, thetubercle having a tubercle width W, the anterior point disposed on thetubercle at a location medially spaced from a midpoint of the tubercleby an amount equal to W/6; providing a tibial prosthesis comprising: adistal surface; a proximal surface generally opposite the distalsurface; and a peripheral wall extending between the distal surface andthe proximal surface, the peripheral wall defining an anterior edge, aPCL cutout area generally opposite the anterior edge, a medialcompartment, and a lateral compartment generally opposite the medialcompartment; and an anteroposterior axis bisecting the anterior edge andbisecting the PCL cutout area; and positioning and orienting the tibialprosthesis with respect to the tibia by aligning the anteroposterioraxis to the anatomic home axis.
 19. The method of claim 18, wherein thetibial prosthesis has a component asymmetry with respect to theanteroposterior axis.
 20. The method of claim 19 further comprising:matching the component asymmetry to a corresponding asymmetry of theresected surface on the proximal end of the tibia, whereby the tibialprosthesis covers between about 60% and about 90% of the resectedsurface.
 21. The method of claim 19, wherein the component asymmetrycomprises the medial compartment being different from the lateralcompartment with regard to at least one of an anteroposterior extent anda peripheral radius.
 22. The method of claim 18, wherein the medialcompartment of the tibial prosthesis cooperates with the anteroposterioraxis of the tibial prosthesis to bound a medial surface area, the latercompartment of the tibial prosthesis cooperates with the anteroposterioraxis to bound a lateral surface area, and the medial surface area islarger than the lateral surface area.